The present invention relates to a process for preparing a foamed article of a vinyl chloride resin containing particles of an inorganic material. According to the process of the present invention, a foamed article generates little heat and produces little smoke and little poison gas at burning can be obtained easily and inexpensively.
Heretofore, a foamed article of a vinyl chloride resin (hereinafter referred to as "PVC") is prepared by the following processes.
(1) High-pressure two-expansion step process
A foamable composition of the PVC, a blowing agent and a stabilizer are packed into a closable mold directly or in a form of a pre-formed material such as a rolled sheet, heated under pressure, and then cooled to a temperature near normal temperature while keeping the inner pressure high to prepare a primary foamed article. The primary foamed article then is taken out from the mold, and heated again with hot air or hot water to carry out secondary expansion.
(2) Crosslinking process
A foamable composition of the PVC, a chemical blowing agent, an isocyanate and an acid anhydride are subjected to the same procedures as in the process (1) to produce a primary foamed article. The primary foamed article is secondarily expanded by heating again with hot water or steam to cause at crosslinking reaction amoung the isocyanate, the acid anhydride and water.
(3) Extrusion process
A composition of the PVC, an acrylic processing aid and a nucleating agent is introduced to an extruder and melted by heating. To the molten composition in the extruder is added a physical blowing agent to give expandability, and then the foamable composition is extruded to carry out expansion.
(4) Elastomer process
A gas-containing PVC plastisol is prepared by mechanically absorbing an inert gas such as carbon dioxide gas into a PVC plastisol with agitation under pressure at a low temperature. The gas-containing PVC plastisol is expanded by high frequency heating method or the like.
According to the conventional processes, a good foamed article containing a large amount of particles of an inorganic material cannot be economically obtained because processability of the composition and the primary foamed article is remarkably lowered.
For instance, in Japanese Examined Patent Publication (Tokkyo Kokoku) Nos. 26776/1977 and 7944/1978, there are disclosed processes for preparing a foamed article of the PVC containing a large amount of the inorganic material according to a modified process of the high pressured two-expansion-step process (1). In those processes, however, since there are employed the fundamental procedure of the process (1) where a primary foamed article is taken out from a cooled mold and then is secondarily expanded by re-heating, temperature inclination, i.e., a temperature near the surface area being high and a temperature being low toward the center, occurs in the foamed article at the secondary expansion step, which results in difference of expansion speed between the surface area and the center area. As a result, the obtained foamed article shrinks and defoams and, in an extreme case, the surface portion is peeled off of itself. Further, a large amount of a cooling medium is required since the primary foamed article must be cooled to normal temperature. In addition, the mold and a press plate used for heating are also cooled as a result of the cooling procedure. Accordingly energy consumption for re-heating the mold and the press plate is increased. Furthermore, this cooling and re-heating cycle takes more time, which results in prolongation of production time and in decrease of productivity. Moreover, considerably more calories are required for the secondary expansion procedure.
As mentioned above, the conventional processes such as the processes described in Japanese Examined Patent Publication (Tokkyo Kokoku) Nos. 26776/1977 and 7944/1978 are insufficient and disadvantageous in productivity, economy and quality of the product.
Also, in the conventional processes mentioned above, the foamable composition or plastisol is prepared by complicated procedures such that two separated kneading procedures are required or kneading procedure must be carried out at an elevated temperature.
An object of the present invention is to provide a process for preparing a high quality foamed article of PVC containing a large amount of particles of an inorganic material economically and with high productivity.